Mongodb $lookup Not working with _id

I finaly found the solution, is a problem with my Schema in mongoose with the ObjectId

I change this

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: String, required: true},
});

with this

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: mongoose.Schema.ObjectId, required: true},
});

and is working

Comparing a string with an ObjectId doesn't throw an error, rather sends an empty array in the aggregated output document. So you need to make sure that you have converted the string object id to mongodb's ObjectId:

db.getCollection('tests').aggregate([
    {$match: {typet:'Req'}},
    {$set: {incharge: {$toObjectId: "$incharge"} }}, // keep the whole document structure, but replace `incharge` into ObjectId
    {$lookup:{
            from: "users",
            localField: "incharge", //this is the _id user from tests
            foreignField: "_id", //this is the _id from users
            as: "user"
    }}
])

First, assert that the type of the incharge field is mongoose.Schema.Types.ObjectId. If you still get an empty array back it might be because you are using the schema name you declared in NodeJS instead of the collection name used by MongoDB.

Example from a UserSchema file:

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const UserSchema = new Schema({
  name: { 
    type: String, 
    required: true
  },
  incharge: { 
    type: Schema.Types.ObjectId, 
    required: true
  },
})

const User = mongoose.model('User', UserSchema)
module.exports = User

The model above is named User by mongoose but the corresponding collection in mongoDB is named users. The resulting $lookup is written as:

$lookup:{
  from: "users",           // name of mongoDB collection, NOT mongoose model
  localField: "incharge",  // referenced users _id in the tests collection
  foreignField: "_id",     // _id from users
  as: "user"               // output array in returned object
}

https://mongoosejs.com/docs/models.html
https://mongoosejs.com/docs/schematypes.html